Electron beam exposure system

ABSTRACT

The invention relates to an electron beam exposure apparatus for transferring a pattern onto the surface of a target, comprising:
         a beamlet generator for generating a plurality of electron beamlets;   a modulation array for receiving said plurality of electron beamlets, comprising a plurality of modulators for modulating the intensity of an electron beamlet;   a controller, connected to the modulation array for individually controlling the modulators,   an adjustor, operationally connected to each modulator, for individually adjusting the control signal of each modulator;   a focusing electron optimal system comprising an array of electrostatic lenses wherein each lens focuses a corresponding individual beamlet, which is transmitted by said modulation array, to a cross section smaller than 300 nm, and   a target holder for holding a target with its exposure surface onto which the pattern is to be transferred in the first focal plane of the focusing electron optical system.

This application claims the priority filing date of U.S. ProvisionalPatent Application No. 60/422,758 filed on Oct. 30, 2002.

BACKGROUND

Several kinds of electron beam exposure systems are known in the art.Most of these systems are provided to transfer very precise patternsonto an exposure surface of a substrate. Since lithography features arepushed to become smaller and smaller following Moore's law, the highresolution of electron beams could be used to continue the drive to evensmaller features than today.

A conventional electron beam exposure apparatus has a throughput ofabout {fraction (1/100)} wafer/hr. However, for lithography purposes acommercially acceptable throughput of at least a few wafers/hr isnecessary. Several ideas to increase the throughput of an electron beamexposure apparatus have been proposed.

U.S. Pat. No. 5,760,410 and U.S. Pat. No. 6,313,476, for instance,disclose a lithography system using an electron beam having a crosssection, which is modified during the transferring of a pattern to anexposure surface of a target. The specific cross section or shape of thebeam is established during operation by moving the emitted beam insidean aperture by using electrostatic deflection. The selected aperturepartially blanks and thereby shapes the electron beam. The targetexposure surface moves under the beam to refresh the surface. In thisway a pattern is written. The throughput of this system is stilllimited.

In U.S. Pat. No. 20010028042, U.S. Pat. No. 20010028043 and U.S. Pat.No. 20010028044 an electron beam lithography system is disclosed using aplurality of electron beams by using a plurality of continuous wave (CW)emitters to generate a plurality of electron beamlets. Each beamlet isthen individually shaped and blanked to create a pattern on theunderlying substrate. As all those emitters have slightly differentemission characteristics, homogeneity of the beamlets is a problem. Thiswas corrected by levelling every individual beam current to a referencecurrent. Correction values for the mismatch are extremely difficult tocalculate and it takes a significant amount of time, which reduces thethroughput of the system.

In Journal of Vacuum Science and Technology B18 (6) pages 3061-3066, asystem is disclosed which uses one LaB₆-source for generating oneelectron beam, which is subsequently, expands, collimated and split intoa plurality of beamlets. The target exposure surface is mechanicallymoved relatively to the plurality of beamlets in a first direction, thebeamlets are switched on and off using blanking electrostatic deflectorsand at the same time scanning deflectors sweep the beamlets which havepassed the blanker array over the target exposure surface in a directionperpendicular to the first direction, thus each time creating an image.In this known system, electrostatic and/or magnetic lenses are used toreduce the image before it is projected on the target exposure surface.In the demagnification process at least one complete intermediate imageis created, smaller than the one before. When the entire image has thedesired dimensions, it is projected on the target exposure surface. Amajor disadvantage of this approach is that the plurality of electronbeamlets together has to pass through at least one complete crossover.In this crossover, Coulomb interactions between electron in differentbeamlets will disturb the image, thus reducing the resolution. Moreover,due to the strong demagnification of the image, the area that is exposedat one time is rather small, so a lot of wafer scans are needed toexpose a die: 16 scans are needed to expose one die, requiring a veryhigh stage speed for reaching a commercially acceptable throughput.

In GB-A1-2,340,991, a multibeam particle lithography system is disclosedhaving an illumination system, which produces a plurality of ionsub-beams. The illumination systems use either a single ion source withaperture plates for splitting a beam in sub-beams, or a plurality ofsources. In the system using a single ion source, the aperture plate isprojected (demagnified) on a substrate using a multibeam optical system.The system furthermore uses a deflection unit of electrostatic multipolesystems, positioned after the multibeam optical system, for correctingindividual imaging aberrations of a sub-beam and positioning thesub-beam during writing. The publication does not disclose how eachsub-beam is modulated. Furthermore, controlling individual sub-beams isa problem, and maintaining inter-sub-beam uniformity.

In Jpn. J. Appl. Phys, Vol. 34 (1995) 6689-6695, a multi-electron bean(‘probes’) lithography system is disclosed having a specific ZrO/W-TFEthermal emission source with an emitter tip immersed in a magneticfield. A disadvantage of such a source is its limited output.Furthermore, this source needs a crossover. The mutual homogeneity ofthe ‘probes’ is not further discussed. Furthermore, the intensity of thesource is a problem

The article furthermore in a general way mentions a writing strategy inwhich a stage, is moved in one direction, and deflectors move the‘probes’ concurrently through the same distance perpendicular to thedirection of the stage movement. A further problem, not recognised inthis publication, is correction of deviation of electron beamlets fromtheir intended positions.

SUMMARY OF THE INVENTION

It is an objective of the current invention to improve the performanceof known electron beam exposure apparatus.

Another objective is to improve the resolution of known electron beamexposures apparatus.

Yet another objective of the current invention is to improve throughputof known electron beam exposure apparatus.

Yet another objective of the current invention is to overcome theproblems related to Coulomb interactions and the demagnification methodsin the prior art.

Another objective of the current invention is to simplify controllinguniformity of beamlets, especially during writing.

The invention relates to an electron beam exposure apparatus fortransferring a pattern onto the surface of a target, comprising:

-   -   beamlet generator for generating a plurality of electron        beamlets;    -   a modulation array for receiving said plurality of electron        beamlets, comprising a plurality of modulators for modulating        the intensity of an electron beamlet;    -   a controller, operationally connected to the modulation array        for individually controlling the modulators using control        signals;    -   an adjustor, operationally connected to each modulator, for        individually adjusting the control signal of each modulator;    -   a focusing electron optical system comprising an array of        electrostatic lenses wherein each lens focuses a corresponding        individual beamlet, which is transmitted by said modulation        array, to a cross section smaller than 300 nm, and    -   a target holder for holding a target with its exposure surface        onto which the pattern is to be transferred in the first focal        plane of the focusing electron optical system.

In this apparatus, electron crossover could be avoided, as it does notdemagnify a complete (part of) an image. In this way, resolution andwriting speed increases. Furthermore, it avoids the needs to control thecurrent in each individual beamlet. The apparatus is less complex as theposition correction and modulation are integrated.

In an embodiment of an electron beam exposure apparatus according to thepresent invention, said modulation array comprises:

-   -   a beamlet blanker array comprising a plurality of beamlet        blankers for the deflection of a passing electron beamlet,    -   a beamlet stop array, having a plurality of apertures aligned        with said beamlet blankers of said beamlet blanker array.

In this way, it is possible to avoid crossover of electron beamlets inone single focal point, and make high-speed modulation possible. In anembodiment, substantially every beamlet blanker is aligned with anelectron beamlet, in order to make it possible to individually modulateevery beamlet. Furthermore, the beamlet stop may comprises at least oneplane of apertures, substantially every aperture being aligned with onebeamlet, preferably with an aperture centred with respect to a beamlet.In this way, a beamlet passes an aperture when an electron beamlet isnot deflected, and a beamlet is blocked or stopped when the beamlet isdeflected. In an embodiment of this modulation array, the controller isoperationally connected to said beamlet blanker.

In an embodiment, the electron beam exposure apparatus is furthermoreprovided with measuring means for measuring the actual position of atleast one of said beamlets, and the controller is provided with memorymeans, for storing said actual position and a desired position, acomparator for comparing the desired position and the actual position ofsaid beamlets, and wherein the adjustor is operationally connected tothe controller for receiving instructions for adjusting the controlsignals issued to the modulators to compensate for the measureddifference between said desired position and said actual position ofsaid electron beamlets. In this way, by adjusting control signals,positioning of the beamlets can be corrected in an easy way. Measurementof the actual positions can for instance be done as described in U.S.Pat. No. 5,929,454.

In an embodiment, the controller is operationally connected to thebeamlet blankers, in an embodiment via the adjustor.

In an embodiment, the adjustor is operationally connected to thecontroller for receiving instructions indicating the amount of theadjustments. The amount of the adjustments can be determined based aresulting value of the above-mentioned comparator.

In a further embodiment, the adjustor is adapted for individuallyadjusting timing of each control signal. In this very easy way,correction can be accomplished.

In an embodiment of the electron beam exposure apparatus according tothe present invention, the beamlet generating means comprise:

-   -   a source for emitting at least one electron beam,    -   at least one beamsplitter for splitting said at least one        emitted electron beam into said plurality of electron beamlets

In this way, a uniform intensity distribution among the beamlets iseasily achieved if the source emits uniformly in all relevantdirections. In an embodiment, the electron beam exposure apparatusfurther comprising a second electrostatic lens array located betweensaid beam splitting means and said beamlet blanker array to focus saidplurality of electron beamlets. In this embodiment, substantially everyelectrostatic lens is aligned and focuses one electron beamlet. In afurther embodiment thereof, the beamlet blanker array is located in thefocal plane of said second electrostatic lens away.

In an embodiment of the electron beam exposure apparatus of the currentinvention with beamsplitter, the beamsplitter comprise a spatial filter,preferably an aperture array. In this way, one source with one beam, or,when source intensity is insufficient or intensity fluctuates across thebeam, several sources are easily split into a plurality of beamlets.

When source intensities are high, the splitting means can comprise anumber of aperture arrays in a serial order along the path of theelectron beam or plurality of beamlets, the aperture arrays havingmutually aligned apertures, each next aperture array along the path fromthe source to the target having apertures that are smaller than theapertures of the previous aperture array. This reduces heat load.

In an embodiment of the aperture array, the apertures of each aperturearray are arranged in a hexagonal structure, which makes it possible toobtain close integration.

In a further embodiment of the electron beam exposure apparatuscomprising splitting means comprising an aperture array, each apertureof the aperture array has an area inversely proportional to the currentdensity based on the beamlet that is transmitted through that sameaperture.

In a further embodiment of the electron beam exposure apparatuscomprising a beamsplitter, the beamsplitter comprises an aperture array,wherein the aperture sizes in the aperture array are adapted to create adiscrete set of predetermined beamlet currents.

These embodiments improve the uniformity of the electron beamlets.

In yet a further embodiment of the electron beam exposure apparatuscomprising the beamsplitter, the beamsplitter comprises an electrostaticquadrupole lens array.

In an embodiment, the electron beam exposure apparatus according to thepresent invention comprises a thermionic source. In an embodiment, thethermionic source is adapted for being operated in the space chargelimited regime. It was found that space charge has a homogenisingeffect, which is favourable in this specific application. Furthermore,in certain settings, the space charge may have a negative lens effect.

In a further embodiment with the thermionic source, the thermionicelectron source has a spherical cathode surface. In an embodiment, thethermionic source comprises at least one extractor electrode. In anotherembodiment, the extractor electrode is a planar extractor electrode. Inan embodiment thereof, the extractor is located after the space chargeregion and provided with a positive voltage for inducing a negative lenseffect. These voltages can be set at a predefined value for creating anegative lens effect for the emitted electron beam.

In an alternative embodiment, the extractor electrode has a sphericalsurface with through holes. All these embodiments serve to create anegative lens influence on the electron beam, thus avoiding a crossoverin the electron beam.

In another embodiment of the electron beam exposure apparatus of thecurrent invention the apparatus further comprises an illumination systemthat transforms the electron beam, emitted by said source, into acollimated electron beam before it reaches said splitting means.

In yet another embodiment of the electron beam exposure apparatus saidbeamlet generator comprises an array of sources of which each source isresponsible for the generation of an electron beamlet. In a furtherembodiment thereof, the electron beam exposure apparatus furthercomprising a second eletrostatic lens array located between said arrayof sources and said beamlet blanker array to focus said plurality ofelectron beamlets.

In an embodiment of the electron beam exposure apparatus with beamletblanking means, said beamlet blanker comprise electrostatic deflectors.

In yet another embodiment of the electron beam exposure apparatusaccording to the invention, it further comprising scanning deflectionmeans provided between the modulation array and the focusing electronoptical system for deflecting the electron beamlets to scan said targetexposure surface. In an embodiment thereof, the scanning deflectionmeans comprises electrostatic scan deflectors. In a fiber embodimentthereof, the electron beam exposure apparatus is further provided withactuating means for moving said electrostatic scan deflectors and saidmeans for holding the target relatively to each other in the plane ofthe surface onto which the pattern is to be transferred in a directionthat differs from the direction of the deflection performed by saidelectrostatic scan deflectors.

In an embodiment, the adjustor or a time shifter are adapted forshifting a timing base of the scanning deflection means and theactuators with respect to each other. In an embodiment thereof, thecontrol signals of tic modulators have a timing base and to actuators ofthe target holder have a second timing base, and there timing bases canbe shifted with respect to one another. This can for instance be used tohave a critical component, which has to be written on the target surfaceand which would lay between two beamlets, written using only onebeamlet.

In a further embodiment thereof, the electron beam exposure apparatusfurthermore comprises an additional aperture plate between themodulation array and the focussing electron optical system, theadditional aperture plate having one surface directed to andsubstantially parallel to the exposure surface of the target, whereinsaid electrostatic scan deflectors are conducting strips deposited onthe side of the additional aperture plate facing the exposure surface ofthe target located between said blanker array and the electrostatic lensarray of the focusing electron optical system. In another embodimentthereof, the electrostatic scan deflectors are conducting stripsdeposited at the target exposure surface side of any of the lens platespresent in the focusing electron optical system. In an embodimentthereof, the conducting strips alternatively have a positive or negativepotential.

In an embodiment of the electron beam exposure apparatus with theblanking electrostatic deflectors, these deflectors deflect the electronbeamlets in such a way that a predetermined section of the beamlet isstopped by the beamlet stop array.

In a further embodiment of the electron beam exposure apparatusaccording to the present invention, it further comprises apost-reduction acceleration stage, located between the electrostaticlens array of the focusing electron optical system and said protectivemeans, for accelerating the electrons in the plurality of transmittedelectron beamlets

In an embodiment of the controller, it is furthermore provided withcorrection means to compensate for the incorrect positioning of theelectron beamlets on the target exposure surface by

-   -   comparing the theoretical position and the actual position of        said beamlets    -   adjusting the control signals to compensate for the measured        difference between said theoretical position and said actual        position of said electron beamlets

In an embodiment of the electron beam exposure apparatus according tothe present invention, it further comprising protective means to preventparticles released by impinging electrons to reach any one of theaperture arrays, lens arrays or blanker arrays, preferably locatedbetween the electrostatic lens array of the focusing electron opticalsystem and the exposure surface of a target, preferably comprising anaperture array wherein the apertures have a size smaller than 20 μm.

In an embodiment of the electron beam exposure apparatus according tothe present invention, all lens arrays, aperture arrays and blankerarrays are connected to a power supply, which, when gas is admitted intothe system, creates a plasma that cleans the plates and removes allcontaminants.

In a further embodiment, the electron beam exposure apparatus accordingto the present invention, the system is operated at an elevatedtemperature of about 200-600° C. to keep the apparatus clean.

The invention further relates to an electron beam exposure apparatus fortransferring a pattern onto the surface of a target, comprising:

-   -   a beamlet generator for generating a plurality of electron        beamlets;    -   a modulation array for receiving said plurality of electron        beamlets, comprising    -   a plurality of modulators for modulating the intensity of an        electron beamlet,    -   a controller, operationally connected to the modulation array,        for individually controlling the modulators using control signs;    -   a focusing electron optical system comprising an array of        electrostatic lenses wherein each lens focuses a corresponding        individual beamlet, which is transmitted by said modulation        array, to a cross section smaller than 300 nm, and    -   a target holder for holding a target with its exposure surface        onto which the pattern is to be transferred in the first focal        plane of the focusing electron optical system,    -   wherein said beamlet generator comprises at least one thermionic        source, said source comprising at least one extractor electrode        adapted for being operated in a space charge limited region,        said source adapted for generating an electron beam, and said        beamlet generator furthermore provided with a beamsplitter for        splitting said electron beam up into a plurality of electron        beamlets.    -   Using such a specific beamlet generator makes it possible to        provide uniform beamlets with a sufficient current to provide a        high throughput.        In an embodiment thereof, said extractor electrode is located        after said space charge region and is provided with a positive        voltage for inducing a negative lens effect to said electron        beam.

The invention furthermore pertains to an electron beam generator forgenerating a plurality of electron beamlets, wherein said beamletgenerator comprises at least one thermionic source, said sourcecomprising at least one extractor electrode adapted for being operatedin a space charge limited region, said source adapted for generating anelectron beam, and said beamlet generator furthermore provided with abeamsplitter for splitting said electron beam up into a plurality ofelectron beamlets.

The invention furthermore pertains to an electron beam exposureapparatus for transferring a pattern onto the surface of a target,comprising a beamlet generator for generating a plurality of electronbeamlets, a plurality of modulators for modulating each electronbeamlet, and a controller for providing each modulator with a controlsignal, said control signal having a timing base, wherein the controlleris adapted for individually adjusting the timing base of a controlsignal with respect to the other control signals.

In this apparatus, the problem of positioning and modulating is solvedin a very simple and elegant way, reducing the number of components andproviding a robust apparatus.

The invention further pertains to a method for transferring a patternonto a target exposure surface with an electron beam, using an electronbeam exposure apparatus described above, and to a wafer processed usingthe apparatus of the curt invention. The apparatus can furthermore beused for the production of mask, like for instance used in state of theart optical lithography systems.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be fib elucidated in the following embodiments of anelectron beam exposure apparatus according to the current invention, inwhich:

FIG. 1 shows an apparatus according to the present invention,

FIG. 2A shows a detail of a known electron beam exposure apparatus,

FIG. 2B shows a detail of the electron beam exposure apparatus,

FIG. 3 shows an electron source with a spherical outer surface,

FIG. 3A shows a source with a space charge region,

FIG. 4 shows an embodiment of a electron beam exposure apparatusstarting from the beamlets,

FIGS. 5A, 5B show embodiments of scan deflection arrays of the currentinvention,

FIGS. 6A, 6B show scan trajectories of the present invention,

FIGS. 7A-7D show adjustment of modulation timing, and

FIGS. 8A, 8B show effects of adjustment of modulation timing.

DESCRIPTION OF EMBODIMENTS

An embodiment of the present invention is schematically shown in FIG. 1.Electrons are emitted from a single, stable electron source 1. Anillumination system focuses and collimates the emitted electron beam 5to illuminate a desired area on an aperture plate 6 uniformly. This canfor instance be established by using lenses 3 and 4. Due to the apertureplate 6 the electron beam 5 is split in a plurality of electronbeamlets, two of which 5 a and 5 b, are shown. An alternative way tocreate a plurality of electron beamlets is to use an array of electronsources. Each electron source generates an electron beamlet, which ismodulated in the same way as the one created with a combination of asingle source and splitting means. Since the emission characteristics ofeach source are slightly different, a single source 1 with beamsplitter6 is preferred. An array of electrostatic lenses 7 focuses each beamletto a desired diameter. A beamlet blanker array 8 is positioned in such away that each individual beamlet coincides with an aperture in the plateof beamlet blanker array 8. The beamlet blanker array 8 comprisesbeamlet-blankers, for instance blanking electrostatic deflectors. When avoltage is applied on a blanking deflator an electric field across thecorresponding aperture is established. The passing electron beamlet, forexample beamlet 9, deflects and terminates at the beamlet stop array 10,located behind the beamlet blanker array 8 following the electronbeamlet trajectory. When there is no voltage applied to the blankingdeflector the electron beamlet will pass the beamlet stop array 10, andreach the focusing electron optical system comprising an array ofelectrostatic lenses 13. This array 13 focuses each of the transmittedbeamlets 12 individually on the target exposure surface 14. Finallyscanning deflection means, most often electrostatic scan deflectors,move the beamlets together in one direction over the target exposuresurface 14. In the embodiment shown in FIG. 1 the scan deflectors arelocated on the target exposure surface side 11 a of beamlet stop array10, thus forming an additional scan deflection array 11. However, otherlocations are also possible. During the scanning the target exposuresurface 14 and the scan deflectors moves relatively to one another in adirection different from the direction of the scan deflection. Usuallythe target is a wafer or a mask covered with a resist layer.

A remarkable aspect of the configuration shown in FIG. 1 is that theentire image that is created by the combination of beamlet blanker array8 and beamlet stop array 10 is not demagnified as a whole. Instead, eachindividual beamlet is individually focused on the target exposuresurface 14 by the focusing electron optical system 13. The differencebetween these two approaches is shown in FIGS. 2A and 2B. In FIG. 2A anentire image comprising 2 electron beamlets 5 a and 5 b is demagnifiedto acquire the desired resolution. To demagnify an image requires atleast one crossing X. In this crossing, all the electrons have to pass asmall area. Coulomb interactions deteriorate the resolution at thatcrossing X.

In the present invention the method shown in FIG. 2B is used. Considertwo adjacent beamlets 5 a, 5 b that are projected on the target exposuresurface 14. Using the demagnification approach the distance between thetwo beamlets also becomes smaller. The focusing approach of the currentinvention, however, does not change this distance between two beamlets.Only the cross section of each beamlet is reduced.

The electron source 1 of FIG. 1 typically delivers 100 A/cm² from anarea of about 30-300 micron squared. In an embodiment, a thermionicsource is used. The electrons are preferably emitted in the space chargelimited emission regime in order to benefit from a homogenizing effectof the space charge. Examples of such a source are a LaB₀ crystal, adispenser source comprising Barium Oxide, or a dispenser sourcecomprising a layer of Barium or Tungsten covered with Scandium Oxide.

The extractor electrodes 2 usually, but not necessarily, focus the beam.The illumination lenses 3-4 create a parallel beam of electrons 5 on theaperture array 6. The lenses 3-4 are optimised to limit the beam energyspread as a result of Coulomb interactions, i.e. the opening angle ofthe beam is made as large as possible. Furthermore lenses 3-4 areoptimised to limit the beam blur created by chromatic and sphericalaberration effects. For the latter it may be advantageous to use theaperture array 6 as a lens electrode, because this may create negativechromatic and spherical aberrations, resulting in a compensation of theaberrations of lenses 3-4. Furthermore, it is possible to use lens 4 formagnification of the pattern by slightly focusing or defocusing it.

In such an embodiment, however, the electron beam emitted from thesingle emitter is focussed in a small crossover x before it is expanded.Within his crossover x there is a large energy spread due toelectron-electron interactions in this crossover x. In the end thecrossover x will be imaged demagnified on the target exposure surface.Due to the Coulomb interactions the desired resolution is not achieved.A method to expand and collimate the expanded beam without a crossoveris therefore desirable.

In a first embodiment, shown in FIG. 3, crossover in the illuminationelfin optics is avoided by using an electron source 1 with a sphericalor a hemispherical outer surface 15. In his configuration a largeopening angle α is formed, which reduces the blur due toelectron-electron interactions in the emitted electron beam 5.Additionally the electron beams are forming a spherical wave front,which results in a virtual crossover 16 located in the centre of thesource. There are no electrons present in the virtual crossover; sodisturbing electron-electron interactions are absent.

The electrons can be extracted with a spherical extractor that compriseslarge holes. The main advantage of the spherical shape of the extractoris the more homogeneous field that is created.

In an alternative embodiment, shown in FIG. 3A, crossover is avoided byextracting the electrons from the source/cathode 1 which is at a voltageVs and has a distant planar extractor 11. The planar extractor has apositive voltage +V₁ with respect to the source 1. The combination ofsource and extractor now serves as a negative lens. The extractedelectrons passing the extractor 1 _(l) thus expand due to the divergingelectric field. Again, a virtual crossover is created, which reduces theloss of resolution due to Coulomb interactions to a great extent.Between source 1 and extractor 1 _(l) a space charged region S ispresent as is shown in FIG. 3A. The presence of this space chargeenhances the negative lens effect created by the source-extractorcombination.

By tuning V₁, it is possible to let the source 1 operate in its spacecharge limited emission mode. The main advantage of this emission modeis the significant increase of homogeneity of the emission. The increaseof the total current can be limited by selecting a source with aconfined emission area.

The aperture array 6 has apertures of typically 5-150 μm in diameterwith a pitch of about 50-500 μm. The apertures are preferably arrangedin a hexagonal pattern. The aperture array 6 splits the incomingparallel beam of electrons 5 in a plurality of electron beamlets,typically in the order of about 5,000-30,000. The size of the aperturesis adjusted to compensate non-uniform current density of theillumination. Each aperture has an area inversely proportional to thecurrent density based on the individual beamlets that is transmittedthrough that same aperture. Consequently the current in each individualbeamlet is the same. If the heat load on the aperture plate becomes toolarge, several aperture arrays are arranged in a serial order withdecreasing aperture diameters along the path of the electron beam orplurality of electron beamlets. These aperture arrays have mutuallyaligned apertures.

Another possible way to split the collimated electron beam 5 into aplurality of electron beamlets is the use of a quadrupole lens array. Apossible configuration of such an array is disclosed in U.S. Pat. No.6,333,508, which document is referenced here as if fully set forth.

FIG. 4 shows a detail closer image of the lithography system in one ofthe embodiments of the present invention starting from the plurality ofbeamlets. Condensor lens array 7 focuses each beamlet to a diameter ofabout 0.1-1 μm. It comprises two aligned plates with holes. Thethickness of the plates is typically about 10-500 μm, while the holesare typically about 50-200 μm in diameter with a 50-500-μm pitch.Insulators (not shown), which are shielded from the beamlets, supportthe plates at typical distances of 1-10 millimetres from each other.

The modulation array comprises a beamlet blanker array 8 and a beamletstop array 10. At the beamlet blanker array 8, the typical beam diameteris about 0.1-5 μm while the typical transversal energy is in the orderof a 1-20 meV. Beamlet blanking means 17 are used so switch the electronbeamlets on and off. They include blanking electrostatic deflectors,which comprise a number of electrodes. Preferably at least one electrodeis grounded. Another eletrode is connected to a circuit. Via thiscircuit control data are sent towards the blanking electrostaticdeflectors. In this way, each blanking deflector can be controlledindividually. Without the use of the beamlet blanking means 17 theelectron beamlet will pass the beamlet stop array 10 through theapertures. When a voltage is applied on a blanking electrostaticdeflector electrode in the beamlet blanker array 8, the correspondingelectron beamlet will be deflected and terminate on the beamlet stoparray 10.

In an embodiment, the beamlet blanker away 8 is located in theelectrostatic focal plane of the electron beamlets. With the blankerarray in hiss position, the system is less sensitive for distortions. Inthis embodiment, the beamlet stop array is positioned outside a focalplane of the electron beamlets.

The transmitted beamlets now have to be focused on the target exposuresurface 14. This is done by a focusing electron optical system 13comprising at least one array with electrostatic lenses. Eachindividually transmitted electron beamlet is focused on the targetexposure surface by a corresponding electrostatic lens. The lens arraycomprises two or more plates 13 a and 13 b, both having a thickness ofabout 10-500 μm and apertures 13 c with a diameter of about 50-250 μm.The distance between two consecutive plates is somewhere between 50-800μm and may be different from plate to plate. If necessary, the focusingelectron optical system may also comprise a lens array of the magnetictype. It is then located between the beamlet stop array 10 and theobjective lens array of the electrostatic type 13, to further enhancethe focusing properties of the electron optical system.

A major problem in all electron beam lithography system patterning awafer or a mask is contamination. It reduces the performance of thelithography system significant due to the interaction between electronsand particles in the resist layer, the resist degrades. In a polymericresist molecules are released due to cracking. The released resistparticles travel through the vacuum and can be absorbed by any of thestructures present in the system.

In order to cope with the contamination problem, in a particularembodiment protective means are located in close proximity of the targetexposure surface, i.e. between the target exposure surface and thefocusing electron optical system. Said protective means may be a foil ora plate. Both options are provided with apertures with a diametersmaller than 20 μm. The protective means absorb the released resistparticles before they can reach any of the sensitive elements in thelithography system. In some cases it is necessary to refresh theprotective means after a predetermined period, e.g. after everyprocessed wafer or mask. In the case of a protective plate the wholeplate ran be replaced. In a particular embodiment, the foil is woundaround the coil winders. A small section of the foil is tightened justabove the tire target exposure surface 14. Only this section is exposedto the contaminants. After a certain period the protective capacity ofthe foil rapidly degrades due to the absorbed particles. The exposedfoil section then needs to be replaced. To do this the foil istransported from one coil winder to the other coil winder, thus exposinga fresh foil section to the contamination particles.

The entire system that is described above operates at relatively lowvoltages. In operations in which high-energy electrons are needed, anadditional acceleration stage is positioned between the electrostaticleas array of the focusing electron optical system 13 and the protectivemeans. This acceleration stage adds energy to the passing electrons. Thebeam may be accelerated additional tens of kiloelectronvolts, e.g. 50keV.

As explained earlier in FIG. 1, the beamlets 12 that have successfullypassed the beamlet stop array 10 are directed towards the desiredposition on the target exposure surface 14 by two means. First of allactuation means move the target exposure surface 14 and the rest of thesystem in a certain mechanical scan direction relatively to each other.Secondly scan deflection means scan the transmitted beamlets 12electrostatically in a direction that differs from the mechanical scandirection. The scan deflection means comprise electrostatic scandeflectors 18. In FIGS. 1 and 3 these scan deflectors 18 are located onan additional aperture array 11, and are depicted in FIG. 4.

In one embodiment, the electrostatic scan deflectors 18 are deposited onthe target exposure surface side of one of the plates of the objectiveelectrostatic lens array 13, such that the deflection essentially occursin the front focal plane of the objective lenses. The desired result isthat the deflected beamlets impinge perpendicularly on the targetsurface.

In another embodiment there are two deflector arrays, one deflecting ina first direction and the other deflecting in a second, oppositedirection. The combined deflection causes displacement of the beamlets adisplacement of the beamlets at the target surface location, withoutchanging the perpendicular axis of a beamlet with respect to the targetsurface.

In a second embodiment, the electrostatic scan deflectors 18 are locatedon the protective means.

The electrostatic scan deflectors 18 comprise scan deflectionelectrodes, which are arranged to deflect an assembly of electronbeamlets in the same direction. The scan deflection electrodes may bedeposited in the form of strips 19 on a suitable plate 20 at the targetexposure surface side as is shown in FIG. 5A. The best yield can beestablished when the strips 19 are deposited close to the beamlet, thusclose to the aperture 21, since this reduces d_(b-sd). Moreover, it ispreferable to position the scan deflection electrodes outside anindividual beamlet crossover plane.

In one embodiment the first assembly is scanned in one direction whilethe next one is scanned in the opposite direction, by puttingalternating voltages on the consecutive strips 19 as is shown in FIG.5B. The first strip has for instance a positive potential, the secondone a negative potential, the next one a positive etc. Say the scandirection is denoted y. One line of transmitted electron beamlets isthen scanned in the −y-direction, while at the same time the next lineis directed towards +y.

As already mentioned there are two scan directions, a mechanical scandirection M and a deflection scan direction S, both depicted in FIGS. 6Aand 6B. The mechanical scan can be performed in three ways. The targetexposure surface moves, the rest of the system moves or they both movein different directions. The deflection scan is performed in a differentdirection compared to the mechanical scan. It is preferablyperpendicular or almost perpendicular to the mechanical scan direction,because the scan deflection length Δx is then larger for the samedeflection scan angle α_(ed). There are two preferable scantrajectories, both shown in FIG. 6 for clarity. The first one is atriangular shaped scan trajectory (FIG. 6A), the second one a saw toothshaped scan trajectory (FIG. 6B).

When the mechanical scan length is a throughput-limiting factor, anassembly of electron beam exposure apparatuses as described above isused to expose the entire wafer at the same time.

It is assumed that an ideal grid exists on the wafer and that theelectron beamlets can be positioned exactly on the grid coordinates. Saythat a correct pattern is created when the electron beamlet can bepositioned within {fraction (1/30)}^(th) of the minimum feature size.Then to write one pixel, 30 scan lines and thus 30*30=900 grid pointsare needed. For the 45 nm-mode the positioning should be controllablewithin a range of 1.5 nm. The data path should therefore be able tohandle an enormous amount of data.

The writing strategy described above is based on the assumption that thebeamlet can only be switched on or off. To reduce the amount of data byless grid lines, and thus less grid cells seems a logical approach.However, the dimension control of the desired pattern suffersconsiderably. An approach to circumvent this problem is to pattern thetarget exposure surface 14 with discrete dose control. Again the patternis divided according to a rectangular grid. However, the number of gridlines is much smaller e.g. 2-5 per dimension, which results in a numberof grid points of about 4-25. In order to get the same patternreliability as for the finer grid, the intensity of each grid cell isvariable. The intensity is represented by a so-called gray value. Incase of a 3 bit gray value representation, the values are 0, {fraction(1/7)}, {fraction (2/7)}, {fraction (3/7)}, {fraction (4/7)}, {fraction(5/7)}, {fraction (6/7)} and 1 times the maximum dose. The number ofdata required for the position of the beamlet reduces, although eachcell is represented with more information duo to the controlled dosevariation.

In the present invention gray scale writing can be introduced in severalways. First of all the deflection of the beams may be controlled in sucha way that part of the beam passes the beamlet stop array 10, while partof the beam continues traveling towards the target exposure surface 14.In this way for instance ⅓ or ⅔ of the beam can be stopped, resulting in4 possible doses on the target exposure surface, namely 0, ⅓, ⅔ and 1times the maximum dose, corresponding to a 2 bit gray valuerepresentation.

Another method to create gray levels is to deflect the beamlets in sucha way that they do not move with respect to the target surface for apredetermined amount of time T, which amount of time T is longer than aminimum on/off time of the blinkers. During time T, the modulator cannow deposite 1, 2, 3, etc. shots on one position, thus creating graylevels.

Another method to create these 4 so-called gray values is to change theaperture size in the aperture array 6. If the are for instance threeaperture sizes, the original size, a size that permits half the originalcurrent to pass and apertures with an area such that only a fourth ofthe original current passes, the same discrete dose values as mentionedbefore an be created. By switching the beamlets on and off with thedeflection electrodes 17 of the beamlet blanker array 8 the desired dosecan be deposited on the target exposure surface 14. A disadvantage ofthe latter method is the fact that more beamlets are needed to write onepixel. Most, including aforementioned methods for discrete dose controlcan also be used to create more than 4 gray values, e.g. 8, 16, 32 or64.

The positions of the beamlets on the target exposure surface most oftendo not exactly correspond with the desired positions. This is forinstance due to misalignment of the different arrays with respect toeach other. Additionally, manufacturing errors may also contribute tothe offset of the individual beamlets. To transfer the correct patternfrom the controller onto the exposure surface of the target, correctionshave to be made. To this end, in a particular embodiment, first theposition of all beamlets is measured and stored. Each position is thencompared to the position the beamlet should have. The difference inposition is then integrated in the pattern information that is sent tothe modulation means.

Since changing the signal sequence that is sent towards the modulationmeans takes a lot of time, the measured difference in position isintegrated in the pattern information by transforming it into acorresponding difference in timing in the beamlet modulation control.FIGS. 7A-7D and 8A-8B explain how the adjustments are implemented. Asalready mentioned the beamlet scan is performed by combining two scanmechanisms: a mechanical scan and a deflection scan. All pattern data,which is sent to each beamlet, is supplied per deflection scan line. Thedesired deflection scan width on the exposure surface of the target thatis patterned, W_(scan), is smaller than the deflection scan width theapparatus can handle, W_(overscan), as is shown in FIGS. 7A AND 7B. Theoverscan ability enables a correction in the deflection scan direction.In FIG. 7A the beamlet is positioned correctly. In FIG. 7B, however, thebeamlet has shifted to the light. By adjusting the timing in such a waythat the pattern data is applied when the beamlet enters the desiredarea, the offset can be compensated for. The adjustment in themechanical scan direction is less precise than depicted in FIG. 7B.Since the pattern generation data is written per scan line, only a disctime delay is possible, i.e. pattern generation can be postponed oraccelerated per scan line. A random time delay would result in acompletely new control data sequence. A calculation of such a newsequence takes a lot of time and is therefore not desirable. In FIGS. 7CAND 7D is depicted what the consequence is. In FIG. 7C again the desiredlocation of the beamlet is shown together with its first fivecorresponding scan lines. In FIG. 7D the real position of the beamletand its trajectories is shown. For clarity the desired beamlet and scanlines are also depicted with an empty circle and dashed lines,respectively. It can be seen that the first scan line in the desiredsituation does not cover the area that needs to be patterned by thebeamlet. So the beamlet start pattering halfway the second scan line.Effectively the delay of information has take a time period that isnecessary to scan one deflection scan line.

FIGS. 8A and 8B show an example of how a change in the timing correctsfor the initial incorrect position of a structure written by a notideally positioned beamlet. FIG. 8A depicts the situation without anytiming correction. The empty dot represents the beamlet at the correctposition, while the filled one represents the real location of thebeamlet. The beamlet is scanned along the drawn line to write a pattern.The line is dashed in the ideal case and solid in the real case. In thisexample the written structure is a single line. Consider a black andwrite writing strategy. i.e. the beamlet is “on” or “off”. The patternis written when the “on” signal is sent towards the modulation means. Inorder to write the single line a certain signal sequence like the oneshown in the upper curve is sent towards the modulation means. When thesame signal sequence is sent in reality, the line is written at adifferent position than desires. The offset of the beamlet leads to anoffset of the written structure.

FIG. 8B shows the situation wherein timing correction is applied. Againthe theoretical and actual spots and trajectories are depicted withdashed and solid lines and dots respectively. The signal sequence in thereal situation is different than the theoretical pattern information, inthe fact that the signal sequence in the real situation (lower curve) issent at a different time than the same sequence is sent in the ideaconfiguration (upper curve). As a result the single line is now writtenat the correct location in the deflection scan direction. Moreover thepattern processing started one scan line earlier resulting in a betterpositioning of the single line in the mechanical scan direction as well.Note that the single line is not precisely positioned at the correctlocation. This is due to the slight offset between the scan lines in theideal and the real situation.

The current electron beam exposure system is thus capable of dynamicallyadjusting the position of a scanned line using timing corrections. Thisallows for critical components in a pattern to be written in one scanline instead of using two halves of two scan lines, which would spreadthe critical component over two scan lines. This correction can also bedone locally, i.e. the timing can be corrected over a small time window.The controller should thus identify critical components, which wouldnormally be spread over two scan lines. Subsequently, the controllershould calculated a corrected timing window, and apply the correctedtiming window to the timing base used for scanning an electron beamlet.FIG. 7D shows the adjustment principle, which could be used for this.

All lens plates, aperture plates and blanker plates can be connected toa power supply, which, when gas is admitted into the system, creates aplasma. The plasma cleans the plates and removes all contamination. Ifone plasma does not clean thorough enough, two gases may be admittedinto the system in series. For instance oxygen may be admitted first toremove all hydrocarbons residing in the system. After the removal of theoxygen plasma, a second plasma, for instance comprising HF, is createdto remove all present oxides.

Another possibility to reduce the contamination is to perform alloperations at elevated temperatures, i.e. 150-400° C. A pretreatment at1000-1500° C. may be necessary. At these temperatures hydrocarbons getno chance to condense on any of the elements in the system. Allowing afraction of oxygen into the system can further enhance the cleaningprocess.

It is to be understood that the above description is included toillustrate the operation of the preferred embodiments and is not meantto limit the scope of the invention. The scope of the invention is to belimited only by the following claims. From the above discussion, manyvariations will be apparent to one skilled in the art that would yet beencompassed by the spirit and scope of the present invention.

1. An electron beam exposure apparatus for transferring a pattern ontothe surface of a target, comprising: a beamlet generator for generatinga plurality of electron beamlets; a modulation array for receiving saidplurality of electron beamlets, comprising a plurality of modulators formodulating the intensity of an electron beamlet; a controller,operationally connected to the modulation array, for individuallycontrolling the modulators using control signals; an adjustor, connectedto each modulator, for individually adjusting the control signal of eachmodulator; a focusing electron optical system comprising an array ofelectrostatic lenses wherein each lens focuses a correspondingindividual beamlet, which is transmitted by said modulation array, to across section smaller than 300 nm, and a target holder for holding atarget with its exposure, surface onto which the pattern is to betransferred in the first focal plane of the focusing electron opticalsystem.
 2. The electron beam exposure apparatus according to claim 1,furthermore provided with measuring means for measuring the actualposition of at least one of said beamlets, and wherein the controller isprovided with memory means for storing said actual position and adesired position, a comparator for comparing the desired position andthe actual position of said beamlets, and wherein me adjustor isoperationally connected to the controller for receiving instructions foradjusting the control signals issued to the modulators to compensate forthe measured difference between said desired position and said actualposition of said electron beamlets.
 3. The electron beam exposureapparatus according to claim 1, wherein the adjustor is operationallyconnected to the controller for receiving instructions indicating theamount of the adjustments.
 4. The electron beam exposure apparatusaccording to any one of claim 1, wherein the adjuster is adapted forindividually adjusting timing of each control signal.
 5. The electronbeam exposure apparatus according to claim 1, wherein said modulationarray comprises: a beamlet blanker array comprising a plurality ofbeamlet blankers for the deflection of a passing electron beamlet, abeamlet stop array, having a plurality of apertures aligned with saidbeamlet blankers of said beamlet blanker array.
 6. The electron beamexposure apparatus according to claim 5, wherein the controller isoperationally connected to said beamlet blankers.
 7. The electron beamexposure apparatus of claim 5, wherein the controller is operationallyconnected to said beam blankers via said adjustor.
 8. The electron beamexposure apparatus according to claim 1, wherein said beamlet generatorcomprises: a source for emitting at least one electron beam, at leastone beamsplitter for splitting said at least one emitted electron beaminto said plurality of electron beamlets.
 9. The electron beam exposureapparatus according to claim 8, wherein said beamsplitter comprise aspatial filter, preferably an aperture array.
 10. The electron beamexposure apparatus according to claim 8, wherein said beamsplittercomprises a number of aperture arrays in a serial order along the pathof the electron beam or plurality of beamlets, the aperture arrayshaving mutually aligned apertures, each next aperture array along thepath from the source to the target having apertures that are smallerthan the apertures of the previous aperture array.
 11. The electron beamexposure apparatus according to claim 9, wherein the apertures of eachaperture array are arranged in a hexagonal structure.
 12. The electronbeam exposure apparatus according to claim 9, wherein each aperture ofthe aperture array has an area inversely proportional to the currentdensity based on the beamlet that is transmitted through that sameaperture, and/or the aperture sizes in the aperture array are adapted tocreate a discrete set of predetermined beamlet currents.
 13. Theelectron beam exposure apparatus according to claim 8, wherein saidbeamsplitter comprises an electrostatic quadrupole lens array.
 14. Theelectron beam exposure apparatus according to any claims 6, 7, and 8,wherein said modulation array comprises a beamlet blander arraycomprising a plurality of beamlet blankers for the deflection of apassing electron beamlet and a beamlet stop array, having a plurality ofapertures aligned with said beamlet blankers of said beamlet blankerarray, said beamlet generator comprises a source for emitting at leastone electron beam and at least one beamsplitter for splitting said atleast one emitted electron beam into said plurality of electronbeamlets, said exposure apparatus further comprising a secondelectrostatic lens array located between said beamsplitter and saidbeamlet blanker array to focus said plurality of electron beamlets. 15.The electron beam exposure apparatus according to claim 14, wherein saidbeamlet blanker array is located in the focal plane of said secondelectrostatic lens array.
 16. The electron beam exposure apparatusaccording to claim 1, wherein said beamlet generator comprises athermionic source.
 17. The electron beam exposure apparatus according toclaim 16, wherein said thermionic source is adapted for operating in thespace charge limited regime.
 18. The electron beam exposure apparatusaccording to claim 16, wherein said thermionic source has a sphericalcathode surface.
 19. The electron beam exposure apparatus according toany one of the claim 17, wherein said beamlet generator comprises atleast one extractor electrode.
 20. The electron beam exposure apparatusaccording to claim 19, wherein said extractor electrode is a planarextractor electrode.
 21. The electron source exposure apparatus of claim20, wherein said extractor electrode is located after said space chargedregion and provided with a positive voltage for inducing a negative lenseffect to said electron beamlets.
 22. The electron beam exposureapparatus of claim 21, wherein said source is adapted for generating anelectron beam and said beamlet generator furthermore comprises abeamsplitter for splitting said electron beam up into said plurality ofbeamlets.
 23. The electron beam exposure apparatus of claim 21, whereinsaid positive voltage is set at a predefined value for creating anegative lens effect for the emitted electron beam.
 24. The electronbeam exposure apparatus according to claim 1, wherein said beamletgenerator comprises a source for emitting at least one electron beam, atleast one beamsplitter for splitting said at least one emitted electronbeam into said plurality of electron beamlets, and an illuminationsystem for transforming the electron beam, emitted by said source into acollimated electron beam before it reaches said beamsplitter.
 25. Theelectron beam exposure apparatus according to claim 1, wherein saidmodulator comprises a beamlet blanker array, wherein said beamletblanker array comprises electrostatic deflectors.
 26. The electron beamexposure apparatus according to claim 1, further comprising scanningdeflection means, preferably provided between the modulation array andthe focusing electron optical system, for deflecting the electronbeamlets to scan said target exposure surface.
 27. The electron beamexposure apparatus according to claim 26, wherein said scanningdeflection means comprises electrostatic scan deflectors.
 28. Theelectron beam exposure apparatus according to claim 27, further providedwith actuators for moving said electrostatic scan deflectors and saidtarget holder relatively to each other in the plane of the surface ontowhich the pattern is to be transferred in a direction that differs fromthe direction of the deflection performed by said electrostatic scandeflectors.
 29. The electron beam exposure apparatus according to claim28, wherein said controller comprises a timeshifter for shifting atiming base of said scanning deflection means and of said actuators withrespect to each other.
 30. The electron beam exposure apparatusaccording to claim 29, furthermore comprising an additional apertureplate between the modulation array and the focusing electron opticalsystem, the additional aperture plate having one surface directed to andsubstantially parallel to the exposure surface of the target, whereinsaid electrostatic scan deflectors are conducting strips deposited onthe side of the additional aperture plate facing the exposure surface ofthe target.
 31. An electron beam exposure apparatus according to claim1, wherein said beamlet generator comprises at least one thermionicsource, said source comprising at least one extractor electrode adaptedfor being operated in a space charge limited region, said source adaptedfor generating an electron beam, and said beamlet generator furthermoreprovided with a beamsplitter for splitting said electron beam up into aplurality of electron beamlets.
 32. An electron beam exposure apparatusaccording to claim 1, further comprising means for transferring apattern onto a target exposure surface.
 33. An electron beam exposureapparatus according to claim 1, wherein the pattern is used fortransferring onto a wafer.
 34. An electron beam exposure apparatus fortransferring a pattern onto the surface of a target, comprising: abeamlet generator for generating a plurality of electron beamlets; amodulation array for receiving said plurality of electron beamlets,comprising a plurality of modulators for modulating the intensity of anelectron beamlet; a controller, operationally connected to themodulation array, for individually controlling the modulators usingcontrol signals; a focusing electron optical system comprising an arrayof electrostatic lenses wherein each lens focuses a correspondingindividual beamlet, which is transmitted by said modulation array, to across section smaller than 300 nm, and a target holder for holding atarget with its exposure surface onto which the pattern is to betransferred in the first focal plane of the focusing electron opticalsystem, wherein said beamlet generator comprises at least one thermionicsource, said source comprising at least one extractor electrode adaptedfor being operated in a space charge limited region, said source adaptedfor generating an electron beam, and said beamlet generator furthermoreprovided with a beamsplitter for splitting said electron beam up into aplurality of electron beamlets.
 35. The electron beam exposure apparatusof claim 34, wherein said extractor electrode is located after saidspace charge region and is provided with a positive voltage for inducinga negative lens effect to said electron beam.
 36. An electron beamexposure apparatus for transferring a pattern onto the surface of atarget, comprising a beamlet generator for generating a plurality ofelectron beamlets, a plurality of modulators for modulating theintensity of each electron beamlet, and a controller for providing eachmodulator with a control signal, said control signal having a timingbase, wherein the controller is adapted far individually adjusting thetiming base of a control signs with respect to the other controlsignals.